Impulse radio communication is radio communication that utilizes a radio frequency (RF) pulse having a tiny pulse width, and is expected to be applied to a broadband radio communication system. Pulse position modulation (PPM) is known as a multivalued modulation technique for the impulse radio communication, which has been developed to increase modulation efficiency. For example, Japanese National Publication of International Patent Publication No. 2003-521143, Japanese Laid-open Patent Publication No. 2000-278332, or Japanese Laid-open Patent Publication No. 2009-88947 discusses a technique for the PPM.
FIG. 1 illustrates an example of the PPM. The PPM is a technique that may delay generation of pulses by delay time dependent on a series of data. When the PPM is performed on a series of 3-bit digital data n1, n2, and n3, generation of the pulses for the series of 3-bit digital data n1, n2, and n3 is delayed by Δt×{22×n1+21×n2+n3}, where nk represents 1 or 0, Δt represents a step time, and Tw represents a time width of an RF pulse. In FIG. 1, a 1-symbol length Ts1, which is the length of time from time t1 to time t3, may be expressed by (23−1)×Δt+Tw, and the transmission speed may be expressed by 3/{(23−1)×Δt+Tw}. That is, the transmission speed in the PPM performed on the n-bit digital data may be expressed by n/{(2n−1)×Δt+Tw}. According to the expression, n/{(2n−1)×Δt+Tw}, efficiency in multivalued modulation may increase as the step time Δt is reduced. However, since the step time Δt depends on the phase identification sensitivity of a device used as a demodulator, the reduction in the step time Δt is limited. For example, when the step time Δt is 20 picoseconds (ps) and the time width Tw is 200 ps in the PPM for the 3-bit digital data, the symbol length Ts1 is 340 ps and the transmission speed is 8.8 gigabits per second (Gbps).